Autonomous regional satellite based navigation systems have enabled some countries to cover their territorial footprint and the footprint of their surrounding areas. A regional satellite based navigation system such as a global navigation satellite system (GNSS) caters to the needs of specific users, for example, military personnel for military applications using a precision service (PS) or a restricted service (RS), and civilian users for civilian applications using a standard positioning service (SPS). The global positioning system (GPS) was declared operational in 1995 and is a dual use system. Prior to May 2000, GPS satellites were transmitting dithered GNSS signals—selective availability (SA), denying civilian users from achieving optimal accuracy. Selective availability was an intentional degradation of public GPS signals implemented for national security reasons. To counter this selective availability issue, a user community developed techniques such as differential GPS (DGPS) to improve accuracy. The DGPS was restricted to a limited area. The success of DGPS drove administrators to rethink selective availability, which was turned off in May 2000. Post removal of selective availability (SA), a major error contributor to a measurement of user position was the ionosphere.
As part of navigation data, a health parameter of each of the satellites is also transmitted. This health parameter is updated once every two hours for the GPS L1 signal. However, if there is any anomaly on board a satellite, the anomaly is only communicated in the next broadcast. With the success of the GPS and the need for correction and/or integrity, a satellite based augmentation system (SBAS) program emerged. Extending the benefits of DGPS, the SBAS became an alternative from a global perspective. The SBAS is a defacto standard used in aviation grade receivers. In addition to accuracy improvement, the SBAS provides integrity information. For aviation applications where the integrity information is of paramount importance, a faster update of the health parameter is required. As a consequence, in addition to other messages, the SBAS provides health information within 6 seconds as part of its navigation data. As part of a receiver user equivalent range error (UERE), the major error component is the ionosphere for single frequency applications. With SBAS corrections, the ionosphere error is minimized. Consumer GPS receivers now support the SBAS, which improves position accuracy.
The GPS aided geo-augmented navigation (GAGAN) system or the GPS and geo-augmented navigation (GAGAN) system is a planned implementation of a regional SBAS by the Indian government. The program currently has one satellite transmitting corrections over India. When fully deployed, the GAGAN system will have three satellites. Currently, one GAGAN satellite is operational. The principle of operation of the GAGAN system is as follows: the GPS satellites visible over the Indian subcontinent are continuously tracked at several stations. These stations are equipped with reference station grade receivers, which provide precise estimates of the pseudorange and carrier phase measurements, typically dual frequency systems. The stations also have antennas located at surveyed locations. With these inputs and traits, measurements are formulated. In addition, these reference station grade receivers also provide estimates on anomalies, if any. The integrity stations are spread across the Indian landmass and all these stations relay data to a master control station. Based on the data collected from various stations, messages are generated. This data is uplinked in C-band to the GAGAN satellite. The GAGAN satellites are configured to transmit signals over the Indian subcontinent in a manner similar to the wide area augmentation system (WAAS). GPS receivers equipped to track the SBAS signals are configured to acquire and track these signals.
In another initiative in space based navigation, India is planning to deploy an autonomous regional satellite based navigation system, namely, the Indian regional navigational satellite system (IRNSS), for surveying, telecommunication, transportation, identifying disaster locations, public safety, etc. The overall constellation of the IRNSS will have seven satellites, three of which will be in geostationary orbits and four of which will be in geosynchronous orbits. Based on open source information, the IRNSS will be a dual use dual frequency system. The signals transmitted by the IRNSS satellites will be in the L5 and S1 band of operation. The civilian signal adopts binary phase shift keying (BPSK) modulation, while a restricted service (RS) will have binary offset carrier (BOC) modulation. The center frequencies will be 1176.45 megahertz (MHz) and 2492.028 MHz. A signal design issue during the initial phases of frequency selection is interoperability with other existing systems, where the signal transmitted by a new system is required to co-exist with existing operational systems. Moreover, from a GNSS serviceability point of view, the signal should typically be in the L band, which is relatively optimal from ionosphere and troposphere related effects. Furthermore, at the time of frequency filing, the signal should be available and should not have been filed by another country. Given the above tradeoffs, the IRNSS signal has evolved with the above recited frequency constraints.
With respect to the control segment, the IRNSS regional integrity monitoring stations (RIMS) will be deployed at several places in the Indian subcontinent. These stations will be equipped with high end receivers which will provide relevant information about the IRNSS satellites. With the signal transmission from the first satellite, these receivers will perform measurements and collect navigation data. The data will be relayed to a master control station. In turn, the master control station will generate Keplerian parameters of all the satellites, clock correction terms, and secondary navigation data information. Unlike the GAGAN system, this is an involved activity that determines the overall system accuracy. The coverage of the IRNSS and the GAGAN system is primarily intended for operation over the Indian subcontinent. In order to improve the accuracy of the IRNSS and to acquire the integrity information, there is a need for an SBAS incorporated IRNSS which reduces payload requirements and avoids the operational overheads required to maintain separate tracking and separate control stations.
A number of issues need to be addressed in the design of a satellite navigation system, for example, sensitivity improvements, jamming margins, robustness towards spoofing, multipath related improvements, time to first fix (TTFF), etc., for ensuring efficiency and robustness of the satellite navigation system. A design parameter that needs to be optimized is the TTFF parameter, which is a measure of time needed by a satellite navigation receiver to acquire satellite signals and navigation data, and calculate a position solution, referred to as a “fix”. The TTFF parameter directly influences the efficiency of position tracking by the satellite navigation receiver. The TTFF parameter is an important receiver specification parameter that serves as a yardstick for comparing satellite navigation receivers from different manufacturers. In order to process a navigation signal emanating from a satellite, a global positioning system (GPS) L1 frequency receiver first establishes a lock on code and carrier frequency. Subsequently, in the lock condition, navigation data from the satellite is demodulated. Conventional satellite navigation receivers require a minimum of four satellites to compute the user navigation solution based on navigation measurements, for example, pseudorange measurements, delta range measurements, etc., and satellite state vectors, for example, the position of the satellite, the velocity of the satellite, etc. For an optimal TTFF performance, there is a need for minimizing the time taken for computing navigation measurements and collecting subsequent navigation data.
A study was carried out on a signaling scheme of operational navigation systems with respect to multiple frequencies of operation. Of all the parameters used to compute TTFF, collection time of ephemeris data (Teph) is a major contributor as Teph completely depends on a navigation data structure of a particular constellation and does not depend on the receiver. In addition, TTFF varies based on various receiver start modes. In general, the start modes can be classified into four categories, for example, a cold start mode, a warm start mode, a hot start mode, and a snap start mode. In the cold start mode, the receiver is powered on without any prior information. This results in more time for computation of the navigation solution as the receiver has to search the signals of all the satellites of a GNSS constellation to obtain a signal lock, demodulate the data bits, and collect the entire navigation data. In the warm start mode, the receiver has access to almanac data, approximate user position and time, which provides an estimate of all the visible satellites. The receiver pre-positions only the visible satellites onto available channels and attempts to acquire the signals. To this extent, the warm start mode differs from the cold start mode, wherein the initial search time to lock on the satellites is reduced. Typically, the TTFF for the cold start mode and the warm start mode is, for example, about 100 seconds and about 48 seconds respectively. The above described start modes are meant for open sky applications.
The next two categories of receiver start modes are the hot start mode and the snap start mode. These start modes are used in automotive grade receivers, wherein the receivers have access to additional parameters. The hot start mode and the snap start mode are used for indoor and high sensitivity applications and are receiver dependent. Specifically, in the hot start mode, the receiver has access to the latest navigation data, that is, ephemeris data, either stored in a memory from the last power-on, or from an external real time aid. As such, the receiver only needs to obtain the time accurately from the satellite. In case of the GPS, a hand over word (HOW) has Z-count information or a time parameter, which repeats once every 6 seconds. Thus, with sub-frame synchronization, the receiver will be able to collect time and in turn make measurements.
The snap start mode is the best case for TTFF, wherein all the receiver parameters including clock parameters of the receiver are available at power on. This category of the receiver makes an assumption that the receiver was recently powered on and that the clock estimate propagated internally is valid for signal processing purposes. With this, the receiver achieves instantaneous lock and with word synchronization, the receiver computes user position. The TTFF for the hot start mode is, for example, about 8 seconds to about 14 seconds, and the TTFF for the snap start mode is 2 seconds. The GPS and the GNSS have civilian ranging codes on dual or triple frequencies. In addition, current receivers have no limitation on the number of channels and thus dual frequency processing has become a defacto standard. With the proposal to deploy the IRNSS over the Indian subcontinent, there is a need for improving the TTFF of the GNSS in the hot start mode and the snap start mode of receiver operation using a data channel on board the geosynchronous satellites of the IRNSS.
Hence, there is a long felt but unresolved need for an SBAS incorporated satellite navigation system that provides a dedicated data channel for improving the TTFF using a third frequency. Moreover, there is a need for a method and system for reducing the TTFF in a satellite navigation receiver operating in the hot start mode and the snap start mode.